Preparation method of electropositive MXene and application of electropositive MXene in salinity difference energy conversion

Abstract

The invention discloses a preparation method of electropositive MXene and application of the electropositive MXene in salinity difference energy conversion. The electropositive MXene is obtained by modifying quaternized xylan nanocrystals obtained through etherification modification by using common waste industrial xylan in the pulping and papermaking field as a raw material, and the electropositive MXene nanofluid membrane has a typical two-dimensional layered structure, has relatively high mechanical strength, shows good anion selectivity in a reverse electrodialysis power generation process, and can be applied to the field of electrodialysis power generation. Good application prospects are shown in the field of reverse electrodialysis.

Description

Technical Field

[0001] The present invention relates to a preparation method of positively charged MXene and its application in salinity gradient energy conversion, belonging to the field of nanofluid salinity gradient energy conversion. Background Art

[0002] The salinity gradient energy obtained from the salinity gradient at the junction of seawater and river water is renewable, easily accessible, and environmentally friendly. It can generate approximately 2.6 TW of energy at the main river estuaries worldwide and is regarded as a rich clean energy source that can meet the growing energy demands of human society.

[0003] Membrane-based reverse electrodialysis technology is the mainstream technology for capturing this clean energy. In particular, nanofluidic membranes self-assembled from two-dimensional materials (such as MXene) are regarded as important materials for capturing salinity gradient energy. However, due to the negatively charged functional groups (such as -OH, -F, =O, etc.) on the surface of two-dimensional materials, in the process of reverse electrodialysis, two-dimensional nanofluid membranes mostly exhibit cation selectivity. Designing environmentally friendly two-dimensional materials with anion selectivity still faces challenges.

[0004] Therefore, there is an urgent need to develop a two-dimensional nanofluid membrane material with low cost and anion selectivity. Summary of the Invention

[0005] In view of the deficiencies of the existing technologies and theories, the present invention proposes a preparation method of positively charged MXene and its application in salinity gradient energy conversion. Using industrial xylan, a common waste in the pulp and paper industry, as a raw material, quaternized xylan nanocrystals are obtained through etherification modification, and two-dimensional material MXene is modified to obtain a positively charged MXene nanofluid membrane. The specific technical solutions are as follows:

[0006] S1: Add 2 - 10 g of quaternized xylan nanocrystals and 1000 ml of deionized water to a 2000 ml beaker, stir at room temperature and 1000 rpm for 2 h to prepare a 0.2 - 1 wt% aqueous solution of quaternized xylan;

[0007] S2: Prepare a solution of high-concentration MXene (about 10 mg mL -1 ) with a concentration of 0.1 - 1 mg mL -1 , stir at room temperature and 500 rpm for 10 min, and use it after uniform dispersion;

[0008] S3: Control the mass ratio of MXene to quaternized xylan, and mix the two solutions prepared in S1 and S2 at different volume ratios;

[0009] S4: Stir at 800 rpm for 24 - 48 h at room temperature;

[0010] S5: The obtained solution was centrifuged at 10000 rpm for 10 minutes. The precipitate was washed twice with deionized water at 7000 rpm for 30 minutes to remove the excess xylan nanocrystals, and the precipitate was taken.

[0011] S6: The obtained precipitate was redispersed in deionized water and sonicated for 5 minutes to obtain a positively charged MXene solution.

[0012] S7: Weigh a certain amount of the positively charged MXene solution and perform vacuum filtration using a 0.22 μm pore size cellulose acetate membrane to obtain a layered positively charged MXene nanofluid membrane.

[0013] In step S1, the preparation method of the quaternized xylan nanocrystals includes the following steps:

[0014] S101: Disperse 5.0 g of xylan in 25 ml of water, add 1.7 g of NaOH, and stir the mixture at 60 °C for 20 minutes to ensure complete dissolution and activation of the xylan.

[0015] S102: Add 15 g of the etherification reagent to the mixed solution in S101 and react at 45 °C for 2.5 hours.

[0016] S103: After the reaction is completed, neutralize with dilute acetic acid and precipitate with ethanol; place the precipitate in a 3500 Da dialysis bag and dialyze for 3 days to remove unreacted reagents or by-products, and then freeze-dry.

[0017] S104: After freeze-drying, cationic xylan nanocrystals are obtained.

[0018] In step S102, the etherification reagent used is one or more of 3-chloro-2-hydroxypropyltrimethylammonium chloride or 2,3-epoxypropyltrimethylammonium chloride.

[0019] In step S2, the preparation method of MXene includes the following steps:

[0020] S201: Take 20 ml of 9 M hydrochloric acid and add it to a 100 ml polytetrafluoroethylene container, add 1.6 g of lithium fluoride, and stir with a magnetic stirrer at 500 rpm for 10 min.

[0021] S202: Weigh 1 g of Ti3AlC2 and add it to the hydrochloric acid / lithium fluoride solution prepared in S201. The reaction temperature is 40 °C and the reaction time is 30 h.

[0022] S203: Add deionized water to the etched mixture and centrifuge at 3500 rpm for 5 minutes to wash away the excess acid. Repeat this step until the pH of the upper layer liquid is neutral, and then collect the precipitate.

[0023] S204: The precipitate was dispersed in deionized water and placed in an ice-water bath for continuous ultrasonic treatment. The total ultrasonic time was 10 min.

[0024] S205: After the ultrasonic treatment, the mixed solution was centrifuged at a rotational speed of 3500 rpm for 30 min to collect the upper-layer liquid, which was the MXene solution.

[0025] In step S3, quaternized xylan nanocrystals and the MXene solution were mixed at different mass ratios, where the mass percentages were (quaternized xylan:MXene) 1:2; 1:1; 2:1 and 3:1.

[0026] The present invention has the following advantages and beneficial effects:

[0027] 1. The raw material used for the quaternized xylan nanocrystals is industrial xylan. This industrial xylan with a linear structure is a by-product obtained in the production of dissolving pulp in the pulp and paper industry and is usually discharged with the waste liquid. It has the advantages of wide sources, low cost, and large output.

[0028] 2. The essence of xylan is a natural polysaccharide, which is ecologically sustainable and has no secondary pollution.

[0029] 3. The introduction of quaternized xylan nanocrystals makes it a surface modifier for MXene, changing the surface electrical property of MXene and forming hydrogen bond linkages with MXene. The prepared positively charged MXene nanofluid membrane has good stability.

[0030] 4. The MXene modified by quaternized xylan nanocrystals has obvious positive charge. The nanofluid membrane constructed by it shows different anion selectivities from other two-dimensional nanofluid membranes, effectively improving the efficiency of osmotic energy conversion.

[0031] 5. It achieves the dual effects of environmental protection and energy recovery. By recycling the by-product industrial xylan in the production process of dissolving pulp, not only the discharged pollutants are reduced, but also the way of high-value utilization of paper-making by-products is expanded. Using quaternized xylan nanocrystals as a modifier for MXene to construct a nanofluid membrane with anion selectivity to improve the conversion efficiency of salinity gradient energy. Description of the Drawings

[0032] Figure 1 It is the SEM photograph and particle size distribution diagram of the quaternized xylan nanocrystals in Example 1 of the present invention.

[0033] Figure 2 It is the TEM photograph of the positively charged MXene prepared in Example 1 of the present invention.

[0034] Figure 3AFM photograph and height map of the positively charged MXene prepared in Example 1 of the present invention. (Optical picture of the nanofluidic membrane prepared in Example 3 of the present invention is as Figure 3 shown)

[0035] Figure 4 Optical picture of the positively charged MXene nanofluidic membrane prepared in Example 1 of the present invention.

[0036] Figure 5 Cross-sectional SEM image of the positively charged MXene nanofluidic membrane prepared in Example 1 of the present invention.

[0037] Figure 6 XRD patterns of the positively charged MXene nanofluidic membrane (bottom) and the original MXene (top) prepared in Example 1 of the present invention.

[0038] Figure 7 Stress-strain curves of the positively charged MXene nanofluidic membrane and the original MXene prepared in Example 1 of the present invention.

[0039] Figure 8 Schematic diagram of osmotic energy conversion of the positively charged MXene nanofluidic membrane.

[0040] Figure 9 Cation transport number and energy conversion efficiency diagrams of the positively charged MXene nanofluidic membrane prepared in Example 1 of the present invention under different salinity gradients.

[0041] Figure 10 Diagram of the relationship between the external output power density, current density and external resistance of the positively charged MXene nanofluidic membrane prepared in Example 1 of the present invention under different salinity gradients.

[0042] Figure 11 Diagram of the relationship between the external output power density, current density and external resistance of the positively charged MXene nanofluidic membrane prepared in Example 1 of the present invention under simulated seawater / river water conditions. Detailed implementation mode

[0043] The following examples are only for further illustration of the technical solution of the present invention, rather than a limitation on the technical solution of the present invention.

[0044] Example 1

[0045] A preparation method of a positively charged MXene nanofluidic membrane, comprising the following steps:

[0046] S1: Add 4 g of quaternized xylan nanocrystals and 1000 ml of deionized water into a 2000 ml beaker, stir at room temperature and 1000 rpm for 2 h to prepare a 0.4 wt% quaternized xylan aqueous solution;

[0047] S2: Prepare a solution of 0.4 mg mL-1 from a high-concentration MXene solution (about 10 mg mL-1), stir at 500 rpm for 10 min at room temperature, and use after uniform dispersion;

[0048] S3: Control the mass ratio of MXene to quaternized xylan, and mix the two solutions prepared in S1 and S2, with the mass percentage being (quaternized xylan: MXene) 1:2; 1:1; 2:1 and 3:1;

[0049] S4: Stir at 800 rpm for 48 h at room temperature;

[0050] S5: Centrifuge the obtained solution at 10000 rpm for 10 minutes. Wash the precipitate twice with deionized water at 7000 rpm for 30 minutes to remove excess xylan nanocrystals, and take the precipitate;

[0051] S6: Redisperse the obtained precipitate in deionized water and sonicate for 5 minutes to obtain a positively charged MXene solution.

[0052] S7: Weigh a certain amount of the positively charged MXene solution and perform vacuum filtration using a cellulose acetate membrane with a pore size of 0.22 μm to obtain a layered positively charged MXene nanofluid membrane.

[0053] It is detected that the concentration of the positively charged MXene solution obtained by the above method is 2 - 5 mg / ml;

[0054] SEM shows that the morphology and size of the prepared quaternized xylan nanocrystals are significantly different from those of industrial xylan, and the nanocrystal size is concentrated in 25 - 45 nm ( Figure 1 );

[0055] Atomic force microscopy (AFM) and field emission scanning electron microscopy (SEM) observe that the prepared positively charged MXene has a clear morphological structure, and its thickness is about 4 nm ( Figure 2 , Figure 3 );

[0056] The digital photo of the obtained positively charged MXene nanofluid membrane is as shown in Figure 4 and presents black with metallic luster;

[0057] The cross-section SEM of the positively charged MXene nanofluid membrane is as shown in Figure 5 and its thickness is 7 μm, with a layer-by-layer self-assembled structure. XRD shows that its layer spacing is about ~2.02 nm ( Figure 6 );

[0058] Example 2

[0059] The positively charged MXene nanofluid membrane prepared in Example 1 was used to detect its mechanical properties and salinity gradient energy harvesting performance.

[0060] As Figure 7 shown, the mechanical properties of the positively charged MXene nanofluid membrane can reach 86.08 MPa, which is 3.5 times that of the original MXene membrane.

[0061] A schematic diagram of the test device for the salinity gradient power generation performance of the membrane material is as Figure 8 shown: Low-salinity and high-salinity sodium chloride salt solutions were introduced into the two electrolytic cells on both sides. Electrodes with silver-silver chloride salt bridges were immersed in the solutions. The positively charged MXene nanofluid membrane was sandwiched between the two electrolytic cells to electrically connect the two electrolytic cells. The temperature of the experimental solution was 25 °C.

[0062] The high-concentration sodium chloride solution was 10 -3 -10 -1 M NaCl solution, and the low salinity was 10 -4 M NaCl solution; the water used was high-resistance pure water with a resistance value of 18.20 MΩ.

[0063] Based on the literature, the cation transference number (t + ) and energy conversion efficiency (η) of the membrane material were calculated by the following equations:

[0064] In the formula, E diff is the diffusion potential; R, T, F, and Z are the universal gas constant, absolute temperature, Faraday constant, and charge number, respectively. λ and c are the ion activity and concentration, respectively.

[0065] As Figure 9 shown, the cation transference number of the positively charged MXene nanofluid membrane is about 0.11, indicating its obvious anion selectivity, and its external output power can reach 28%.

[0066] According to the formula P = I 2 ×R, the external output power of the positively charged MXene nanofluid membrane was calculated.

[0067] As Figure 10 shown, as the salinity gradient increases, the output power density increases. At a 1000-fold concentration gradient, its external output power can reach ~0.5 W m -2 indicating its good osmotic energy conversion performance.

[0068] 0.5 M and 0.01 M brines were prepared to simulate seawater and fresh water in nature, and the actual salinity gradient energy generation efficiency of the positively charged MXene nanofluid membrane was detected.

[0069] As Figure 11As shown, the output power of the positively charged MXene nanofluid membrane can reach 3.02 W m -2 .

[0070] In summary, the positively charged MXene nanofluid membrane proposed in the present invention changes the ion selectivity of the original two-dimensional nanomaterials, enabling anions to be transported along the ion transport channels, thereby generating electrical energy and showing great application potential

[0071] It should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Those of ordinary skill in the art should understand that based on the above description, other different forms of changes can be made, and obvious modifications or changes to the technical solutions of the present invention, without departing from the spirit and scope of the technical solutions of the present invention, should all be covered within the protection scope of the present invention.

Claims

1. A method for preparing positively charged MXene and its application in salt difference energy conversion, characterized in that: The following steps are involved: S1: Add 2-10 g of quaternized xylan nanocrystals and 1000 ml of deionized water into a 2000 ml beaker, stir at room temperature and 1000 rpm for 2 h to prepare a 0.2-1 wt% quaternized xylan aqueous solution; S2: Prepare a high concentration MXene solution (about 10 mg mL-1) into a 0.1-1 mg mL-1 solution, stir at room temperature and 500 rpm for 10 min, and use it after it is evenly dispersed; S3: Control the mass ratio of MXene to quaternized xylan and mix the two solutions prepared in S1 and S2 in different volume ratios; S4: Stir at 800 rpm for 24 to 48 h at room temperature; S5: The obtained solution was centrifuged at 10000 rpm for 10 minutes. The precipitate was washed twice with deionized water at 7000 rpm for 30 minutes to remove excess xylan nanocrystals, and the precipitate was taken; S6: The obtained precipitate was redispersed in deionized water and ultrasonically treated for 5 minutes to obtain a positively charged MXene solution. S7: Weigh a certain amount of positively charged MXene solution and perform vacuum filtration, using a cellulose acetate membrane with a pore size of 0.22 μm to obtain a layered positively charged MXene nanofluid membrane.

2. The method for preparing a positively charged MXene and its application in salt difference energy conversion according to claim 1, characterized in that: In step S1, the method for preparing quaternized xylan nanocrystals comprises the following steps: S101: 5.0 g of xylan was dispersed in 25 ml of water, 1.7 g of NaOH was added, and the mixture was stirred at 60 °C for 20 minutes to ensure that the xylan was completely dissolved and activated; S102: adding 15 g of etherification reagent to the mixed solution in S101, and reacting at 45° C. for 2.5 hours; S103: After the reaction is completed, the mixture is neutralized with dilute acetic acid and precipitated with ethanol; the precipitate is dialyzed in a 3500Da dialysis bag for 3 days to remove unreacted reagents or by-products, and then freeze-dried; S104: After freeze-drying, cationic xylan nanocrystals are obtained. Wherein, in step S102, the etherification agent used is one or more of 3-chloro-2-hydroxypropyltrimethylammonium chloride and 2,3-epoxypropyltrimethylammonium chloride.

3. The method for preparing a positively charged MXene and its application in salt difference energy conversion according to claim 1, characterized in that: In step S2, the method for preparing MXene includes the following steps: S201: Take 20 ml of 9M hydrochloric acid and add it to a 100 ml polytetrafluoroethylene container, add 1.6 g of lithium fluoride, and stir with a magnetic stirrer at 500 rpm for 10 min; S202: Weigh 1 g of Ti3AlC2 and add it to the hydrochloric acid / lithium fluoride solution prepared in S201. The reaction temperature is 40°C and the reaction time is 30 h. S203: adding deionized water to the etched mixed solution and centrifuging at 3500 rpm for 5 min to wash away excess acid, repeating this step until the pH of the upper liquid is neutral, and collecting the precipitate; S204: The precipitate is dispersed in deionized water and placed in an ice water bath for continuous sonication for a total of 10 minutes; S205: After the ultrasound is completed, the mixed solution is centrifuged at a rotation speed of 3500 rpm for 30 minutes to collect the upper liquid, which is the MXene solution.

4. The method for preparing a positively charged MXene and its application in salt difference energy conversion according to claim 1, characterized in that In step S3, quaternized xylan nanocrystals and MXene solution are mixed in different mass ratios, wherein: the mass percentages are (quaternized xylan:MXene) 1:2; 1:1; 2:1 or 3:

1.

5. The method for preparing a positively charged MXene and its application in salt difference energy conversion according to claim 1, characterized in that: The positively charged MXene nanofluid membrane provides an ion transport channel between high-concentration salt solution and low-concentration salt solution.

6. The method for preparing a positively charged MXene and its application in salt difference energy conversion according to claim 1, characterized in that: High concentration NaCl concentration is 10 -3 ~5×10 -1 M, the NaCl concentration on the side with low electrolyte concentration is 10 -4 ~10 -2 M.

7. The method for preparing a positively charged MXene and its application in salt difference energy conversion according to claim 1, characterized in that: Positively charged MXene nanofluid membranes are anion selective.

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